1. Field of the Invention
The invention relates generally to sonic (e.g., ultrasound) methods of making suspensions of uniform, non-agglomerated particles, and more particularly to methods for making a chemical reaction product that is a suspension, dispersion or emulsion of non-agglomerated, uniformly shaped particles at high production rates of up to 100 gallons per minute or higher. When the chemical reaction is effected in the presence of sonic energy, in close proximity to the point of contact of the reactants in the reactor, intimate mixing of the reactants is achieved to facilitate a more complete, uniform reaction than is achieved using conventional bladed-mixer systems.
2. Description of the Prior Art
Suspensions of very small, solid or liquid particles are useful in many applications, including personal care products (e.g., shampoos, soaps, etc.), cleaning products, paints, coatings, foodstuffs, fertilizers, pool chemicals, and the like. Generally, a well-dispersed suspension or emulsion of uniformly sized, non-agglomerated particles is desired because such suspensions provide a large and uniform surface area which results in improved performance of these products. Accordingly, much effort has been expended to develop methods to prepare well-dispersed suspensions of uniformly sized, non-agglomerated small particles, particularly such particles in the range of from submicron size to a few microns in size. One method involves using chemistry to control the particles"" size and/or shape during their formation in the chemical reaction employed to produce the particles. Another method uses physical mixing of the particles, during or after their production in a liquid suspension or emulsion, or grinding of the particles that are formed, to provide a desired size or shape. As another alternative, a combination of these methods has been investigated heretofore.
Chemical methods for control of particle size and/or shape generally focus upon manipulating the parameters of the precipitation reaction under which the particles are formed. For example, the use of additives, such as surfactants, to the particle-forming precipitation reaction mixture is suitably utilized to provide a suspension of specific shaped particles having a particle size distribution within the range of from about 0.5 to 30 microns (micrometers or xe2x80x9cxcexcmxe2x80x9d) in size. However, it is difficult to achieve suspensions of small solid or liquid particles having an xe2x80x9cessentially uniform size and shapexe2x80x9d using surfactant additives alone. The term xe2x80x9cessentially uniform sizexe2x80x9d as used herein, is intended to designate that the particles referred to have dimensions that do not vary by more than twenty percent, preferably not more that ten percent, between individual particles in the particulate product. The term xe2x80x9cessentially uniform shapexe2x80x9d, as used herein, is intended to designate that the particles referred to have essentially identical shapes, i.e., that the shapes of the particles within a given particle distribution are essentially identical. More specifically, if the particles in a distribution referred to as xe2x80x9cessentially uniform in shapexe2x80x9d are largely hexagonal in shape, then at least 80%, preferably at least 90%, of the particles in this distribution would be hexagonal in shape.
By changing other reaction parameters, such as by decreasing the temperature of the precipitation reaction, in combination with the use of a surfactant additive, it is possible to produce suspensions of solid particles having dimensions in a particle size range of from 0.5 to 5 xcexcm. However, this range of size distribution is still greater than might otherwise be desired. Accordingly, there is a continuing need in the dispersions, suspensions and emulsions manufacturing community for particulate products having a particle distribution that is essentially uniform in size and shape. The present invention provides one answer to that need.
Heretofore, various mixing and/or grinding techniques have been employed in an effort to further reduce particle size without impairing the configuration or shape of the particle. Generally, conventional mixing procedures utilize a blade-type mixing apparatus such as a blender. The blade portion of the apparatus rotates at a specified rate to generate shear forces that physically reduce the sizes of the particles. Unfortunately, however, these bladed mixers pose a number of problems in the manufacture of suspensions of small particles, such as biocides. For example, bladed mixers tend to pull air into the reaction medium, and the entering air can cause unwanted foaming or thickening of the suspension. Blade-type mixers also have the undesirable effect of providing non-uniform mixing at various points within the reaction chamber. This result is believed to be attributable to the fact that the amount of shear force generated at the edge of the blade is greater than elsewhere in the reaction vessel, such as the surface, bottom or sides of the reactor vessel. Needless to say, differing applied shear forces at different points within the reactor vessel can adversely influence the chemistry of particle formation.
In view of these problems and disadvantages, it is difficult to prepare a well-dispersed suspension of uniformly sized and shaped, non-agglomerated particles using a bladed mixing approach. Nonetheless, in the production of solid biocides, bladed mixing, grinding, and centrifugation have found widespread use. For example, the biocides iodopropargylbuylcarbamate (so-called xe2x80x9cIPBCxe2x80x9d) and pyrithione are typically produced using bladed mixing or centrifugation, and the resulting product is generally size- and shape-determined by virtue of the reactants and reaction parameters that are employed. In the past, biocide manufacturers have used grinding to further reduce the size, or alter the shape, vis-à-vis the size and shape of the particles that result from the reaction itself. Unfortunately, however, grinding tends to have an adverse effect on the desired uniformity of shape of the biocide particles, as discussed in more detail hereinbelow.
Specific examples of useful pyrithione biocides include polyvalent metal salts of pyrithione (also known as 1-hydroxy-2-pyridinethione; 2-pyridinethiol-1-oxide; 2-pyridinethione; 2-mercaptopyridine-N-oxide; pyridinethione; and pyridinethione-N-oxide). These pyrithiones have enjoyed widespread application as fungicides and bactericides in paints and personal care products such as anti-dandruff shampoos. The polyvalent metal salts of pyrithione are solids that are only sparingly soluble in water and include magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione. The most widely used divalent pyrithione salts are zinc pyrithione and copper pyrithione. Both zinc and copper pyrithione are useful as antimicrobial agents active against gram-positive and negative bacteria, fungi, and yeasts. Zinc pyrithione is used as an antidandruff component in shampoos, while technical suspensions of zinc pyrithione and/or copper pyrithione are used as preservatives in paints and polymers. Synthesis of polyvalent pyrithione salts are described in U.S. Pat. No. 2,809,971 to Berstein et al. Other patents disclosing similar compounds and processes for making them include U.S. Pat. Nos. 2,786,847; 3,589,999; 3,590,035; and 3,773,770.
The size limitations on pyrithione salt particle production made by conventional bladed mixing methodology demonstrate the drawbacks of using such processing. Illustratively, known methods for producing insoluble polyvalent salts of pyrithione typically result in solid particles having an average size greater than one micrometer (xcexcm). However, as discussed above, smaller particles of pyrithione salts (i.e., less than one micron in size) are often desired because they more easily form suspensions and provide a larger surface area for enhanced biocidal activity. In addition, smaller particles, particularly in the low submicron range (e.g., below about 0.1 or 0.2 xcexcm), may be transparent to light, and thus could provide the opportunity to manufacture xe2x80x9cclearxe2x80x9d products, such as clear shampoos and soaps, that are popular in the marketplace today.
Smaller particles of pyrithione salts are usually generated by a separate mechanical manipulation step (e.g., grinding or crushing) on larger particles or crystals that are made by conventional processes. For example, European Patent Application No. 70046 describes preparation of zinc pyrithione using organic solvents. This process results in production of large crystals of zinc pyrithione that are easily isolated by filtration. A separate, optional grinding step is used to grind the large crystals and produce zinc pyrithione particles of smaller size. U.S. Pat. No. 4,670,430 describes a process of making zinc pyrithione particles with a median size of about 0.2 xcexcm by grinding larger particles of zinc pyrithione.
As an alternative to grinding, sonication technology has been used to break up large solid or liquid particles into smaller ones. To date, however, this technology has not been utilized to enhance commercial chemical reactions, to the knowledge of the present inventors, for a variety of reasons, such as scale-up problems discussed below. Nevertheless, the recent technical journal literature provides laboratory-scale data suggesting that sonication holds promise for facilitating high product yield from chemical reactions under mild reaction conditions while promoting a short reaction time. The facilitation of chemical reactions using sonication in industry could provide a major commercial advantage, such as in the manufacture of biocides such as zinc pyrithione or IPBC, both in terms of enhanced product throughput and in terms of minimization of product damage by using mild processing conditions.
Classic applications of sonication for commercial purposes have included xe2x80x9cphysicalxe2x80x9d treatments, such as cleaning, drilling, emulsification promotion, soldering, sonar detection, medical therapy, and welding. Ultrasonic sound waves have also been used commercially to analyze and evaluate the physical and chemical properties of materials, such as density, porosity, viscosity, and chemical composition.
Another field of technology has claimed attention recently; termed xe2x80x9csonochemistryxe2x80x9d, whereby ultrasound has been applied to mediate laboratory-scale organic synthesis reactions, as documented in Jean-Louis Luche""s treatise xe2x80x9cSynthetic Organic Sonochemistryxe2x80x9d (Plenum Publishing Corporation, 1998). Challenges are faced, however, in efforts to scale-up such laboratory sonochemical methods, as described at page 326 of Luche""s text wherein the states that xe2x80x9cinitial steps for the successful industrial set-up of sonochemical procedure are the geometrical problems (reactor design) associated with mapping and solution measurementsxe2x80x9d. In a similar vein, a technical journal article by Frerich J. Keil and Sascha Daehnke published in the Hungarian Journal of Industrial Chemistry, vol. 25, no. 1, pp. 71-80 (1997) points out scale-up issues based upon energy density distribution and cavitation bubble calculations in the reactor. In the face of these scale-up issues and problems, sonochemical methodology has not been applied to its full commercial advantage heretofore, to the knowledge of the present inventors.
In view of the above, there is clearly a need in industry, that has not been satisfied to date using sonochemical methodology, for a commercially utilizable method for rapidly preparing a suspension, dispersion, or emulsion of non-agglomerated particles of a desired size and shape. Ideally, such a method would produce particles of an essentially uniform size and shape under mild reaction conditions, while avoiding the harsh shear conditions associated with bladed mixers and grinding machines. The present invention provides one answer to this need.
In one aspect, the invention is directed to an improved method for making a suspension, emulsion or dispersion of non-agglomerated solid or liquid particles, comprising the step of forming the particles by a chemical reaction of at least two reactants in a liquid medium in the presence of sonic energy, wherein the improvement comprises applying said sonic energy to the liquid medium at the point of contact of the reactants with each other, thereby causing intimate mixing of the reactants and an associated rapid completion of the reaction to produce a desired product while minimizing or reducing the risk of forming unwanted byproduct, said desired product being in the form of said suspension, emulsion or dispersion of non-agglomerated particles having an essentially uniform size and shape.
In another aspect, the present invention relates to a continuous method of making a suspension, emulsion or dispersion of non-agglomerated solid or liquid particles on a commercial scale, said method comprising continuously forming the particles by a transchelation reaction of at least two reactants by contacting said reactants at a point of contact, and applying sonic energy at said point of contact in order to cause intimate mixing and facilitate rapid transchelation at said point of contact, thereby producing a desired product while minimizing or reducing unwanted byproduct production, said desired product being in the form of said suspension, emulsion or dispersion of non-agglomerated particles resulting from said transchelation.
In another aspect, the invention is directed to a method for making a suspension of non-agglomerated pyrithione salt particles by forming pyrithione salt particles in a liquid medium by a transchelation reaction of at least two reactants, wherein sonic energy is applied to the liquid medium during the forming step at the point of contact of said reactants in order to produce the suspension of non-agglomerated pyrithione salt particles.
In yet another aspect, the invention is directed to a method for making a suspension of non-agglomerated particles of pyrithione salts, comprising the step of reacting pyrithione or a water-soluble salt of pyrithione and a water-soluble polyvalent metal salt in an aqueous medium and in the presence of a dispersing additive to produce particles of pyrithione salts, wherein sonic energy is applied to the aqueous medium during the reacting step to produce the suspension of non-agglomerated pyrithione salt particles, the non-agglomerated pyrithione salt particles having a median size of from about 0.01 to about 50 microns. The xe2x80x9cdispersing additivexe2x80x9d is suitably a dispersant or other agent (e.g., a thickening agent such as carboxymethylcellulose, so-called xe2x80x9cCMCxe2x80x9d) that facilitates the formation of the desired stable suspension of solid or liquid particles in the aqueous medium.
These and other aspects will become apparent upon reading the following detailed description of the invention.